1. Field of the Invention
The present invention relates to a technique for controlling an automatic transmission including a mechanical clutch and a mechanical transmission and, in particular, to a technique for protecting a mechanical clutch when a vehicle moves backward in a so-called hill start.
2. Description of the Related Art
In recent years, automatic transmissions have been proposed (refer to Japanese Unexamined Patent Publication No. 2001-165294), that are configured to electronically control a mechanical clutch and a mechanical transmission (this type of transmissions will be hereunder referred to as “mechanical automatic transmissions”), thereby automatically changing gear to a drive stage according to traveling conditions. In a mechanical automatic transmission, since there is no fluid clutch (torque converter) in the driving force transmission system from the engine to the driving wheels, the driving force transmission efficiency is high, so that it is possible to improve fuel consumption. Furthermore, since there is no slipping sensation, which is peculiar to a fluid clutch, it is also possible to improve the drivability.
Incidentally, when starting on an uphill, when the brake is released from a departure standby state in which the transmission is shifted to the forward stage and the mechanical clutch is disengaged, there is a possibility that the vehicle moves backward due to gravity. At this tine, if the accelerator pedal is pressed down to start moving without noticing the vehicle moving backwards, since the input shaft of the transmission is reversed, the mechanical clutch is engaged in a state in which the relative rotational speed is high. Thus, there is concern that the mechanical clutch might be damaged. Conversely, on a downhill, when the brake is released from a departure standby state, there is a possibility that the vehicle is moved forward due to gravity. At this time, if the accelerator pedal is pressed down to start moving from a state in which the vehicle speed is high, the mechanical clutch is engaged in a state in which the rotation speed of the input shaft of the transmission is higher than the engine rotation speed. Therefore, there is concern not only that the mechanical clutch might be damaged, but that the engine be over-revolved.
In order to prevent such an undesirable situation from occurring, conventional techniques have prevented damage to the mechanical clutch by shifting the transmission to the neutral at the point in time when the vehicle speed reaches a predetermined value while in a departure standby state. Then, when the accelerator pedal or shift lever is operated after the transmission is shifted to the neutral, the transmission shifts automatically to the drive stage according to the operation, and starts driving straight away.
However, since the gearshift is performed according to the operation of the accelerator pedal or the shift lever, when the vehicle moves backward on an uphill, the mechanical clutch is sometimes engaged as before in a state in which the relative rotational speed is high. Therefore, there is concern that the mechanical clutch gets damage. There is also concern that such an undesirable situation may similarly occur, when an attempt is made to start the vehicle in reverse on a downhill.
Furthermore, a typical rotational speed sensor detects rotational speed by detecting the teeth of a gear using an electromagnetic pick up or a Hall element. In this case, since the rotational speed sensor detects only the teeth of the gear, it can only measure the absolute value of the rotational speed regardless of the direction of rotation. As a result, in the case where an occurrence of a reverse rotation of the transmission is to be determined using the rotational speed sensor, two rotational speed sensors must be installed for the same gear with their positional phases shifted, and the existence of a reverse rotation must be determined from the phase difference of their output signals. However, this method requires space to install the two rotational speed sensors, and installation accuracy, and also requires an electric circuit for exclusive use to perform signal processing with high accuracy. Hence, there is a problem of cost, and furthermore the burden on the control software is large.